There exists in the United States today a renewed interest in the development of highly efficient external heat engines similar to the engine disclosed by Robert Stirling in 1816 and built in 1827. This engine is very simple in principle of operation, being no more than the tendency of a gas to expand when heated. Useful work or shaft power output can be derived from this expansion process. The Stirling engine cycle, which uses a regenerative heat exchange system, is known to be more efficient than either the Otto or Diesel cycles and can approach the theoretical limits of thermal efficiency as described by the well-known Carnot cycle. Also, a reciprocating piston, Stirling engine structure which uses a regenerative heat exchange system can be operated in reverse, that is to say, it can be driven by another power source, such as a Stirling engine, to make it an effective heat pump or refrigerator system.
The basic Stirling engine, and any other conventional heat engine for that matter, is comprised of a thermal energy source, a thermal energy sink (usually the atmosphere), and a means for converting available heat energy into useful mechanical energy. The heart of the Stirling engine, and most other external heat source engines, is in the ability and capability of the thermal management system to efficiently transport and exchange thermal energy available from the source to the sink.
Thermal management systems for Stirling-type heat engines and heat pumps are usually comprised of a working fluid capable of transporting thermal energy and generating working pressures, a heat exchanger component for energy input from the thermal source, a "regenerator," defined here as a device for rapid reversible thermal energy storage and recovery relative to said working fluid, and a heat exchanger component for energy rejection to the thermal sink. The efficiency and cost of heat exchangers and regenerators are of primary importance for the successful design of Stirling and other external-heat engines.
Present state-of-the-art heat exchanger system designs for reciprocating piston Stirling engines such as the United Stirling 4-95 are typically comprised of three basic components. The first component is a heat input heat exchanger which consists of parallel arrangements of high-temperature metal alloy tubes which may also be attached or welded to many heat fins or heat sinks to provide a larger convective and radiative area for heat exchange; the second component is a regenerator which consists of an enclosed in-line stack of fine mesh stainless metal screens; and the third component is a heat output heat exchanger which consists of an enclosed annular duct internally containing an arrangement of many metal fins which may be attached to a water-cooled outer wall. Said metal tubes for heat exchangers are typically composed of high-temperature, high-strength alloys containing strategic heavy elements, such as niobium, titanium, tungsten, cobalt, vanadium, and chromium, in addition to iron and carbon. This use of strategic elements drives up the basic material costs. The use of strategic metal alloys also drives up the cost of fabricating the parts due to the requirement for using non-standard and high-temperature forming methods. The heat exchanger system alone may account for 10 to 100 times the cost of all other components combined in state-of-the-art Stirling engines. The prohibitive cost, bulk, and weight of the state-of-the-art heat exchanger systems are the primary factors limiting the wide scale commercial development of external-combustion heat engines and refrigerator systems.
Stirling and other external-combustion heat engines which rely on a substantially closed loop arrangement of a conductive gas or multiphase fluid are particularly sensitive to the conditions of flow which exist throughout the heat exchange loop. The cross-sectional area and shape of the heat exchanger inlet and outlet ports are important design parameters which govern to a large extent the flow characteristics of a fluid under given pressure and temperature state variables which typically exist in reciprocating and free piston heat engines. As a rule of thumb, the cross-sectional area of the orifices through which the working fluid or heat energy transport medium must flow should be high relative to the cross-sectional area of the piston in order to achieve a relatively low Reynolds number or flow index. Competing with this is the desire to minimize the total volume of fluid participating in the heat exchange cycle and the desire to maximize the surface area available for the thermal energy exchange process which occurs between the working fluid and the walls of the flow passageways. State-of-the-art metal tubes tend to be few in number due to the high cost of the tubes, and each tube tends to have a small diameter, resulting in a low cross-sectional area. The low cross-sectional area in state-of-the-art heat exchangers causes adverse flow conditions for the primary working fluid flowing through the heat exchanger system, resulting in poor thermal efficiencies and drastically reduced engine performance compared to model predictions. Increasing the diameter of each tube to reduce the flow velocity results in reduced heat transfer of the fluid to the walls of the tube. Conversely, decreasing the diameter of the tubes to increase the heat transfer efficiency results in increased fluid velocity for a constant number of tubes. As the working fluid is caused to ingress and egress the heat exchanger orifices, the velocity of the the working fluid approaches the sonic velocity limits, resulting in reduced heat transfer efficiency due to the restriction of the total amount of fluid which may flow through the heat exchanger system. Another effect of sonic-limited flow is to cause significantly reduced power output of the engine since no useful work can be derived from the trapped working fluid both before or aft of the heat exchanger orifices.
A practical heat exchanger design is bounded by parameters seeking to maximize the thermal energy transfer rate and capacity, and to minimize the pressure, velocity and temperature of the working fluid consistent with the structural and thermal properties and loadbearing capability of the heat exchanger materials and components.
As gas working fluid expands or compresses through an orifice and connecting passageways of constant or varying cross section dimensions, energy is transferred between the walls of the chamber and the gas molecules. The characteristics of the energy transfer process occurring between the working fluid and the walls of the flow passageway are dependent on the thermodynamic conditions of the expansion or compression process (i.e., adiabatic, isothermal, isobaric, isentropic) and on the flow characteristics (i.e., laminae, turbulent, or transition) and boundary layer development near the walls of the flow passageway. The thermal efficiency of the heat exchanger is defined in terms of the capability to rapidly transfer heat energy between a working fluid medium and an external heat source and heat sink.
Regenerator effectiveness is generally defined in terms of the temperature difference which accompanies the heat transfer process between the working fluid and the walls of the regenerator. The sensitivity of the Stirling engine to the effectiveness of the regenerative component of the heat exchanger system is illustrated as follows: reducing the regenerator efficiency by two percent reduces the efficiency of the engine by approximately four percent. This is due to the fact that if the regenerator efficiency is reduced by two percent, then the extra quantity of heat must be made up by the input heat exchanger and by the heat output exchanger. Since the heat output is generally fixed by the available thermal sink temperature, the heat input exchanger makes up the total difference by operating at a higher temperature, which requires more fuel input while the shaft power output remains constant. This reduces the total efficiency of the engine for a given shaft power output. State-of-the-art regenerators consist of costly in-line stacks of fine mesh, stainless metal screens. Other regenerator designs have been tried, but the stacked metal screens have shown the highest regenerator effectiveness due to the associated high flow rates (velocity) of the working fluid.
Instead of a stack of fine mesh metal screens, the present invention uses a stack of thermally conductive and thermally insulating layers in alternating relation. The layers have communicating holes therethrough in a central area and have an outer nonperforated area to serve as a thermal reservoir in the case of the intermediate thermally conductive layers. The two outer layers are thermally conductive; one is heated outside of the central area and the other is cooled over most of its outer face. The intermediate thermally conductive layers take on heat energy from fluid passing from the hot to the cool end of the heat exchanger and release heat energy to fluid passing in the reverse direction. Such a stack of alternating layers will hereinafter be referred to as "SAL." The communicating holes through the layers provide continuous passageways through the stack. Preferably, the holes alternate in size from layer to layer to provide multiple expansion chambers along the length of each passageway.
This invention aims to improve the overall performance and thermal efficiency for Stirling and other heat engines by increasing the total orifice cross-sectional area and simultaneously increasing the surface area available for heat transfer in the flow passageways while maintaining structural reliability and safety. Increasing the orifice area effectively reduces the Reynolds numbers or flow characterization indices of the working fluid medium contained by the heat exchanger system and, in particular, reduces the Reynolds numbers in the regenerator. As an example, the heat exchanger section used in a single Stirling 4-95 engine cylinder is comprised of 18 tubes, each being 3 mm in diameter, for a total cross-sectional area of the heat exchanger orifice of (127.23 mm 2) compared to a piston area of (2375.82 mm 2), which is a ratio of only (0.0535) or 5.35% of the total piston area. In contrast, the heat exchanger of this invention can be made such that the total entrance port area of the orifices equals a cross-sectional area of 50.0% of the total piston area and, furthermore, accomplish this by providing many more flow passages, which can be much smaller (1 mm diameter), resulting in greater heat transfer efficiency. The flow rates are greatly reduced due to the larger total cross-sectional orifice area and the gas working fluid can flow more easily through the heat exchange system. Furthermore, the flow passageways of the heat exchanger disclosed in this invention may be given a total length which is comparable to the stroke of the piston travel of the engine rather than several times this stroke length as compared to the use of metal tubes. This shorter flow path length results in less trapped gas working fluid and hence increased heat exchange efficiency.
The regenerator and heat input and output exchangers must be efficient due to the frequent flow reversals which may occur in an engine during operation. For example, at an engine crankshaft rotational speed of 3000 rpm or 50 Hertz, the entire cycle time for heat transfer into and out of the gas working fluid occurs within 0.02 seconds. Thus a very short time interval is available during which the gas working fluid must accomplish the heat exchange process. The efficiency is governed in part by the thermal conductivity of the gas working fluid.
A high-power and efficient Stirling engine using air as a gas working fluid is highly desirable. Hydrogen and helium are two of the most thermally conductive dry gases, being approximately nine times more conductive than dry air. However, air saturated with water vapor as a gas working fluid exhibits high thermal conductivity comparable to helium, but is more viscous and is constrained to move at a slower bulk velocity. The heat exchanger system disclosed in this invention allows wet air to be efficiently used as a gas working fluid in a Stirling engine due to the large frontal orifice area of the heat exchanger flow passageways relative to the piston face area.
Another object of this invention is to significantly reduce the overall weight and dimensions of the Stirling and other heat engines using a SAL heat exchanger as compared to state-of-the-art engines using the relatively heavy, lengthy, and bulky parallel arrangements of finned, strategic metal alloy tubes. The weight of the regenerator and heat exchanger components is determined by the product of the value of the mass density of the materials in the respective components and the value of the heat capacity of said materials consistent with temperature variations allowed in the thermal management system. By the present invention, the thermal load capacity of a heat exchanger may be increased or decreased simply changing the number of layers in the stack and by increasing the dimensions of the perimeter or nonperforated region of said layers.
A still further objective of this invention is to reduce the cost of the regenerator components by replacing the costly stainless metal screens in state-of-the-art regenerators with a relatively low-cost, stacked, alternating layers regenerator while still maintaining a high regenerator effectiveness due to the reduced flow rates (velocity) of the working fluid in the regenerator. In the preferred embodiment of this invention, the regenerator stack serves to locally and rapidly store and recover heat energy from the working fluid and to thermally insulate the heat input heat exchanger which is continuously supplied heat energy from an external heat source from the heat output heat exchanger which is continuously expelling heat energy to an external heat sink. The hole patterns in the stacked, alternating layers are arranged such that the gas working fluid alternates between local compression and expansion chambers in the flow passageways. This is accomplished by simply alternating the hole diameters in adjacent layers in the regenerator, thereby forming localized chambers in the flow passageways. As the gas is caused to ingress into a larger chamber, expansion occurs; and as the gas egresses to the next smaller chamber, compression occurs. This localized compression/expansion process occurs continuously as the working fluid flows through the heat exchanger and regenerator and acts to increase the rate of heat transfer between the working fluid and the walls of the flow passageways. This reduces the amount of nonparticipating or adiabatic working fluid contained in the center of the flow stream and acts to substantially improve the overall efficiency of the engine or the heat pump.
A still further objective of this invention is to increase the capability of the Stirling-type engine to use many types of heat energy sources and sinks including radioactive sources. This is made possible because all of the layers of the heat exchanger can be or ceramic materials which are adapted for use in a radioactive environment.
This invention also aims to balance or uniformly distribute the temperature gradients existing near the reciprocating piston face opposite the heated, outside, thermally conductive layer of the SAL. State-of-the-art metal tube designs position the metal tubes of the heat exchanger in a line across the face of the piston, resulting in nonuniform temperature gradients both radially and circumferentially about the cylinder axis. The orifices of each flow passageway existing in each layer of the heat exchanger as described by this invention are more evenly distributed across the face of the piston, thus acting to uniformly distribute the temperature of the gas flowing in the heat exchanger.
A yet further objective of this invention is to substantially reduce the hoop stress loads due to pressure and to improve the safety and reliability of high-temperature and high-pressure heat exchanger and regenerator components. The hoop stresses are safely mitigated in the layered heat exchanger structure by simply increasing the outer dimension or diameter of each layer. In the event that a single flow passageway wall cracks or fails, there will not be any resulting leakage or catastrophic failure of the system unless the crack extends completely through to the exterior of the entire layer structure. It is also well known in brittle failure theory that each hole of a pattern of small holes contained by a structure and subject to positive internal pressure loads will each act individually as stress risers. However, a crack trying to propagate through the entire structure will be deflected by the small holes and will have its propagation energy absorbed by said holes which are contained in the structure, thus acting to inhibit crack tip propagation and thus act to prevent catastrophic failure of the heat exchanger. Hence the SAL heat exchanger of this invention has a higher safety factor as compared to state-of-the-art, tube-type heat exchangers.